This invention relates to a piston-cylinder kit for internal combustion engines, particularly for diesel engines, which kit is designed for a top land clearance of 0.3 to 1.5%, preferably of 0.4 to 1.0%, of the piston diameter, and in which the sliding surface of the cylinder has, adjacent to the combustion chamber, a peripherally extending profiled marginal portion.
For an economical operation, an internal combustion engine should have a low fuel consumption and also a low oil consumption although the latter is only a fractional part of one percent of the fuel consumption. An increase of the oil consumption will not only reduce the economy but will also result in an undesired deposition of carbon in the combustion chamber and on the piston and in the valves and this may result in considerable consequential damages. A high oil consumption will also increase the emission of pollutants into the environment. On the other hand an excessive decrease of the oil consumption may adversely affect the operation of the internal combustion engine by resulting in an excessive wear or in a seizing of the piston. Because the oil consumption of internal combustion engines, particularly of diesel engines, should not be in excess of 0.5 g/kWh, considerable efforts are made at present to properly design and match the piston and to provide an optimum design for the piston rings and for the sliding surface of the cylinder. In that connection a special problem is presented due to the so-called polishing of the cylinder because this results in an increase of the oil consumption with the operating time of the piston and an increase of the danger of a seizing of the piston after a long operating time.
The polishing of the cylinder is effected by hard oil carbon which has been deposited on the top land of the piston during the operation of the engine and which during the longitudinal and transverse motion of the piston smoothens the honed profile formed on the sliding surface of the cylinder. In that case the lubricating oil can no longer accumulate in the scores which have been honed into the sliding surface of the cylinder having a depth of 2 to 10 micrometers, but is displaced from that sliding surface into the combustion chamber and is burnt therein. This results in a deficiency of oil, so that the wear of the piston rings and the formation of scores therein will be increased. Besides, small cracks will be formed in the sliding surface of the cylinder where it has been polished and the piston rings cannot closely fit the sliding surface of the cylinder at said cracks so that the scraping action of the piston rings is distinctly reduced and the oil consumption is increased.
From SAE Paper No. 845,010 it is known that the polishing of the sliding surface of the cylinder will be reduced or eliminated if the clearance at the top land amounts to 1.5 to 2% of the piston diameter. But that advantage cannot be afforded unless a higher temperature of the piston is tolerated. For instance, in a diesel engine the temperature will be higher by 30.degree. to 50.degree. C. at the rim of the combustion chamber recess and by 20.degree. to 30.degree. C. at the first piston ring or at the first piston ring groove. Besides, the fuel consumption will increase by an order of 2% and pollutants will be emitted at a higher rate. Whereas the increase of the piston temperature can reasonably be offset by an effective cooling of the piston and the increase of the fuel consumption and of the pollutant content of the exhaust gas can reasonably be offset by a decrease of the axial height of the top land from the usual height of 15 to 18% of the piston diameter to a height of 8 to 10% of said diameter, the results which can thus be produced are not satisfactory in view of the requirements to be met by modern internal combustion engines.
For this reason it has been proposed to subject the cylinders of diesel engines in a marginal portion which is adjacent to the combustion chamber to a non-cutting shaping so as to form said marginal portion with such a profiled shape that the building up of a layer of hard oil carbon on the top land can be prevented or kept within such narrow limits that there will be no excessive increase of the oil consumption and of the emission of pollutants in the course of the operation of the internal combustion chamber. By the measure stated above the hard oil carbon which has deposited on the top land will continually be removed by the piston as it continually engages and clears the peripherally extending, profiled upper portion of the sliding surface of the cylinder so that the polishing of the cylinder will be restricted to a permissible extent. But in such a cylinder the crests of the profile obtained by the non-cutting shaping protrude beyond the main portion of the sliding surface of the cylinder by about 0.2 mm so that the piston must be inserted into the cylinder when the latter has not yet been installed. Another disadvantage of that proposal resides in that the accuracy of the profile thus obtained is reproducible only with relatively large tolerances.